Allow externally defined KoFu DSLs

[0n3z]

AbstractDsl initialize() is marked as internal. This prevents external sub classes to work properly. If the constraint is relaxed, it would allow external projects to provide their own applicative DSLs or side projects to add featured DSLs enabling fast community driven enrichment.

[sdeleuze]

Good point, I will change that as part of 0.3.

[sdeleuze]

I will then rollback #218 and make it public since protected does not allow the DSLs to initialize each others. Maybe #275 will allow extensibility while not exposing such "internal" method on the public API cc @poutsma.